This invention relates to radar retroreflectors, and particularly, to retroreflectors which control the polarization of reflected electromagnetic radiation.
Radar systems detect a "target" such as an aircraft by irradiating an area with electromagnetic radiation from a radar transmitter and analyzing the radiation reflected back to a radar receiver to ascertain the presence of the target. When reflective surfaces other than the target are present, they produce reflected waves called "clutter." Clutter may be caused by many things including weather conditions, such as ice crystals, rain drops, hail stones, etc. When an aircraft is immersed in rain, it is often difficult to discriminate the presence of the aircraft target from the rain clutter.
Circular polarization is sometimes employed in radar systems to distinguish targets from clutter. By transmitting circularly polarized electromagnetic waves and receiving only waves having the same sense of circular polarization (clockwise or counterclockwise) as the transmitted wave, it is possible to suppress reflections caused by rain clutter. This is accomplished by using receiving equipment which is sensitive only to waves having the same sense of polarization as that of the transmitted wave and insensitive to waves having the opposite sense of polarization.
It is well understood in the art that radiation reflected from rain clutter will have a reverse sense of circular polarization. In contrast, a target such as an aircraft will reflect substantial radiation having the same sense of circular polarization as the transmitted waves. Circularly polarized waves reflected by rain clutter are received having a sense of polarization which is opposite to the sense of polarization of the transmitted waves. However, a substantial portion of circularly polarized waves reflected from targets is received having the same sense of polarization as the transmitted waves. Thus, suppression of rain clutter is accomplished by rejecting waves having a reverse sense of polarization from the transmitted waves.
The same-sense radar return from most perspectives of an aircraft is acceptable for use with a radar system employing circular polarization to discriminate targets from clutter. Even though an aircraft is a complex target with many equivalent corners, circularly polarized waves doubly bounced from corners will return with the same sense of polarization, so aircraft returns typically undergo but little suppression. However, viewed nose-on the same-sense radar return from an aircraft is weak, therefore it is desirable to enhance this return.
One known way of enhancing the cross-section of a target is by the use of a retroreflector. A retroreflector is an apparatus which receives an electromagnetic radiation wave from a source and returns the received wave back toward the source. A corner reflector which resembles a corner cut off from a cube is one type of retroreflector. In general, electromagnetic radiation incident on such a corner reflector bounces three times and is reflected back substantially in the opposite direction to the direction of arrival. Retroreflection takes place with a corner reflector over a broad range of incident angles so that the corner reflector does not have to be aligned with the source. The reflection from a circularly polarized incoming radar wave, however, will be polarized in the opposite sense to that transmitted since it will have been reflected three times within the corner. Thus, a rain rejection radar which rejects oppositely polarized waves will also reject waves reflected from a corner reflector.
A dipole antenna can also be used to return an enhanced reflection wave. However, if circular polarization is employed, only half of the returned radiation will have the same sense of polarization. Also, the returned radiation from a dipole is not retroreflected, but scattered in nearly all directions.
Retroreflection can be achieved by arranging a group of dipoles in a conventional linear or planar van-Atta array configuration to concentrate the return energy back towards the source. The conventional van-Atta array works well with waves polarized in a single plane. But circularly polarized waves retroreflected from a van-Atta array will have only half of the reflected radiation polarized in the same sense as the impinging radiation. Thus, the conventional van-Atta array may provide less enhancement than is desired.
A need exists for a method and an apparatus for enhancing the radar cross-section of a target to circularly polarized radar waves which provide retroreflection while at the same time controlling the sense of circular polarization of the returned waves, so rain clutter suppression can be used effectively.